Dark Matter

Posted 06.25.08

NOVA scienceNOW

Host Neil deGrasse Tyson reports from a half mile underground in an abandoned mine, where scientists are using special detectors to look for evidence of a ghostly substance that they believe makes up most of the matter of the universe—a hypothetical entity called dark matter.

Transcript

DARK MATTER

PBS Airdate: June 25, 2008

NEIL
deGRASSE TYSON: Hi. I'm Neil
deGrasse Tyson. Welcome to a new season of NOVA scienceNOW.

Now,
I'm an ordinary guy, and that means, of course, I'm made up of
ordinary matter: basically, atoms. And when we gaze out into space, everything
we see—galaxies, stars—is also ordinary, made of atoms.

But
a lot of scientists say there's something else in the universe
that's NOT ordinary.

Wait,
who said that?

NEIL'S
JACKET SLEEVE: And by the way, there seems
to be way more of this weird stuff than ordinary guys like you...

NEIL
deGRASSE TYSON: Hey, watch it!

And
even though it's invisible, it's getting harder and harder to
ignore.

Every
day, a crew squeezes into an 80-year-old elevator in Minnesota and commutes to
work a half a mile down, into the depths of an abandoned mine.

They're
not searching for gold or diamonds. Instead, they're mining for something
even more coveted and harder to find, something called dark matter.

RICHARD
MASSEY: Dark matter is one of the biggest
mysteries.

TALI
FIGUEROA: Dark matter is
everywhere.

RICHARD
MASSEY: We wouldn't be here if it
weren't for the dark matter. Life wouldn't be possible.

TALI
FIGUEROA: The problem is
we have no clue what the dark matter is.

JOCELYN
MONROE (Massachusetts Institute of
Technology): We know it's
out there, and we just have to find it.

NEIL
deGRASSE TYSON: One of the people
now trying to find dark matter is physicist Tali Figueroa.

TALI
FIGUEROA: Discovering
dark matter is going to be one of the greatest finds of the century.

NEIL
deGRASSE TYSON: So, they really
mine iron in this place.

His
search takes place a half-mile under ground, where this old iron mine has been
transformed into a cavernous, space-age physics lab.

When
I visited, I didn't notice any dark matter, but I did see quite a bit of
dead matter.

Whoa,
what is this thing?

TALI
FIGUEROA: That's a
bat.

NEIL
deGRASSE TYSON: It doesn't
look very alive.

TALI
FIGUEROA: Probably isn't.

NEIL
deGRASSE TYSON: Whoa, there's
one there...another, another.

TALI
FIGUEROA: They're
all over the place.

NEIL
deGRASSE TYSON: That's nasty.

TALI
FIGUEROA: It is, kind
of.

NEIL
deGRASSE TYSON: Nasty. So that
doesn't creep you out?

TALI
FIGUEROA: You get used
to it.

NEIL
deGRASSE TYSON: Down here,
surrounded by the dead bats, Tali and his colleagues monitor and care for a
complex contraption specially designed to detect particles of dark matter.

So
this is it, huh?

TALI
FIGUEROA: Yup.

NEIL
deGRASSE TYSON: This elaborate
endeavor is all to solve a mystery that's been plaguing astrophysicists
for more than 70 years.

It
might seem bizarre and even a bit crazy, but there's a chance that most
of the matter in the universe is not stars or planets or gas or anything familiar
to us, but is in the form of some mysterious invisible substance. We've
labeled it "dark matter," but why do we think it exists at all?

It
comes down to gravity and speed. Ever since Isaac Newton, we've known
that it's gravity that holds objects in orbit, just as the sun holds
Earth and the rest of the planets.

The
stronger the gravity pulling it inward, the faster an object can go and stay in
orbit. It's kind of like spinning a heavy ball around: the harder you
pull on the ball, the faster the ball will travel. If the ball gets moving too
fast, even a strong guy like this has got to let go.

PETER
FISHER (Massachusetts Institute of
Technology): The faster you want
something to go—like, you know, David throwing his slingshot—the
more you have to pull on it. And the thing that's pulling on something to
make it orbit is gravity.

NEIL
deGRASSE TYSON: And where does
gravity come from? Well, we know it can be things with mass like stars, houses,
planets, trains, clouds, jellyfish; they all have gravity.

So,
in the universe, the more stuff, the more gravity, and the faster objects can
move and remain in their orbits. The problem is when we look out beyond our
solar system, like at stars orbiting within galaxies, or galaxies moving within
galaxy clusters. They're all orbiting faster than we'd expect.

JOCELYN
MONROE: The speed at which the stars are
going around at is too fast. You would expect that it should just escape, but
those stars don't escape. They're still going around.

NEIL
deGRASSE TYSON: There's got
to be a lot of gravity holding them all together, but apparently there's
not enough matter to account for it.

PETER
FISHER: And
there's not enough stuff. There's just not enough stuff to keep
them all going around each other.

NEIL
deGRASSE TYSON: Regardless of how
we probe the cosmos for this missing matter—using visible light, radio
waves, x-rays—we still come up short. Either we've got the laws of
gravity completely wrong, or there's got to be more stuff. Actually, we'd
need about five times more stuff. It's stuff we can't see, but what
exactly is it?

RICHARD
MASSEY: What is dark matter?

MAX
TEGMARK (Massachusetts Institute of
Technology): What is the dark
matter?

RICHARD
MASSEY: Yeah, that is a big question.

TALI
FIGUEROA: We don't
know what it is.

RICHARD
MASSEY: It's completely invisible.

TALI
FIGUEROA: It's
dark. It doesn't glow.

MAX
TEGMARK: So,
whatever the dark matter is...

TALI
FIGUEROA: We can't
point a telescope up and actually see it.

MAX
TEGMARK: ...it
sure ain't made of atoms.

NEIL
deGRASSE TYSON: Everything around
us that we can see and touch, ordinary matter, is made of atoms. But one thing
we know is dark matter is not ordinary.

RICHARD
MASSEY: We know its not ordinary matter,
because ordinary matter has all this whole other variety of interactions. It
has electric fields and magnetic fields. It emits light.

NEIL
deGRASSE TYSON: One idea is, since
it's not made of ordinary atoms, dark matter might be made of some exotic
particle. Right now, physicists around the world are racing to build a detector
sensitive enough to capture one, so they can figure out exactly what it is.

But
how do you catch a particle that's so shy?

TALI
FIGUEROA: The
fundamental problem is that this dark matter does not interact with matter very
much. And so, in order to detect it, we have to build these really specialized,
very sensitive detectors.

NEIL
deGRASSE TYSON: At this underground
lab, Tali Figueroa is monitoring one kind of dark matter detector, a
superconducting crystal made from the element germanium.

So,
one of your detectors, huh?

TALI
FIGUEROA: Yes, this is a
prototype of one of the 30 detectors. And when you look at the surface of our
detector, you'll see a metal grid.

NEIL
deGRASSE TYSON: The grid picks up
tiny temperature changes, produced when a particle hits the crystal and sets
all its atoms vibrating. But to detect those vibrations, the atoms in the
crystal have to start out as still as possible, something atoms don't
normally like to do.

TALI
FIGUEROA: The problem is
that naturally, at room temperature, the atoms are vibrating themselves.

NEIL
deGRASSE TYSON: So, how does the
team manage to slow down the detector's atoms? They put it in a freezer,
a very powerful freezer.

So
the whole point of this is to simply keep the experiment cold?

TALI
FIGUEROA: Yes. We have
to keep the experiment at about 50 milliKelvin, which is 50/1000 of a degree
above absolute zero.

NEIL
deGRASSE TYSON: Just a fraction of
a degree above absolute zero? Translated into Fahrenheit, that's, like,
460 degrees below zero. So, in other words, it's cold enough so that the
air we breathe freezes solid.

TALI
FIGUEROA: Absolutely.

NEIL
deGRASSE TYSON: And so
there's frost everywhere.

But
now there's another problem. The frozen detector is so hyper-sensitive,
lots of things could set it off, like cosmic rays, particles that shower Earth
from space. So this is why the whole lab is deep under ground.

So
the bedrock...

TALI
FIGUEROA: The half a
mile of rock...

NEIL
deGRASSE TYSON: ...above...

TALI
FIGUEROA: ...is a
shield.

NEIL
deGRASSE TYSON: ...is a shield. So
the cosmic rays...these are high energy particles from space?

TALI
FIGUEROA: From space.

NEIL
deGRASSE TYSON: Okay. So
you're protecting yourself from space.

And
it's not just cosmic rays. Even under ground, there are other tiny
particles flitting around us, including photons and neutrons that can fly out
of the surrounding rock. So the detectors are cloaked in layer upon layer of
shielding, all in an effort to filter out everything but the dark matter. And
how are things going so far?

Okay,
how many dark matter particles have you found so far?

TALI
FIGUEROA: None.

NEIL
deGRASSE TYSON: None?

TALI
FIGUEROA: None.

NEIL
deGRASSE TYSON: It's not too
surprising. The quest for dark matter here on Earth has only just begun, and
bigger and more sensitive detectors are already in the works. Still, you might
wonder, could it be that dark matter is something that's just out there
in space and not down here with us?

Astrophysicist
Richard Massey says, "not likely." He's got the first-ever,
3-D dark matter maps to back him up. But how do you map the unseeable?

RICHARD
MASSEY: So we can't see dark matter
directly; it's completely invisible. But we can work out where it is by
its effects on the ordinary matter that we can see.

NEIL
deGRASSE TYSON: In other words, you
can see dark matter's gravity. That's because, according to
Einstein and nearly a century of experiments, what gravity does in the universe
is bend space. Massive objects like the sun actually bend and stretch the
contours of space. That's what keeps smaller objects, like Earth, in
orbit.

And
if space is bent, so is any light that passes through it.

RICHARD
MASSEY: So let's debunk the whole
idea that light travels in straight lines. Light travels in what it thinks are
straight lines. And because space is warped and bent, even the straight lines
that light rays travel along are actually bent themselves.

NEIL
deGRASSE TYSON: The phenomenon is
called gravitational lensing. Think of what a thick magnifying glass can do the
text of a book.

RICHARD
MASSEY: When we put a magnifying glass in
front of it, we start seeing a distorted image, and gravitational lensing to
find dark matter works in a very similar way.

NEIL
deGRASSE TYSON: A huge clump of
dark matter and the enormous gravity it creates would bend areas of space so
much, it would act like a giant cosmic lens, distorting our view of distant
galaxies.

The
more distortion, the more gravity, and, Massey assumes, the more dark matter
lies between them and us.

RICHARD
MASSEY: So, the final result is that we
end up having this map of where the dark matter is in the universe.

NEIL
deGRASSE TYSON: Maps such as these
are now revealing that galaxies like ours are completely enveloped by giant
clouds of dark matter.

RICHARD
MASSEY: Wherever there's ordinary
matter, so even here, there is some dark matter. It's everywhere. The two
really have gone together, hand in hand.

NEIL
deGRASSE TYSON: In fact, as the universe
evolved after the Big Bang, dark matter may have served as a kind of
cosmological glue that, over time, helped pull stars together to form galaxies.

RICHARD
MASSEY: We owe everything to dark matter,
in two ways: firstly, it holds the whole universe together; but then it also,
crucially...inside that, it forms this scaffolding in which the ordinary matter
can lay to grow.

MAX
TEGMARK: We are so
lucky to have dark matter, because we wouldn't even be here otherwise. It
was the gravitational attraction from dark matter that pulled together this
diffused gas that eventually formed our Milky Way galaxy that we live in. And
if there were no dark matter, then our galaxy would, in fact, never have
formed.

NEIL
deGRASSE TYSON: If that's
true, then it's not just our Milky Way. Across the universe, none of the
billions of galaxies out there would have formed without the gravity of this
mysterious stuff.

Now,
we just need to find out what it is.

MAX
TEGMARK: It's
really astonishing that there's five times more stuff out there than we
know of, and that we've been at this, as a community, for over 70 years. And
yet it might be now, in the next few years, that we'll figure it all out.
It's just incredible.

This material is based upon work supported by the National
Science Foundation under Grant No. 0638931. Any opinions, findings, and
conclusions or recommendations expressed in this material are those of the
author(s) and do not necessarily reflect the views of the National Science
Foundation.

Funding for NOVA scienceNOW is provided by the National Science Foundation, the Alfred P. Sloan Foundation, and PBS viewers.

National corporate funding for NOVA is provided by Cancer Treatment Centers of America.
Major funding for NOVA is provided by the David H. Koch Fund for Science, the Corporation for Public Broadcasting, and PBS viewers.